1. Field of Invention
This application relates to test equipment and more specifically to test equipment measuring characteristics of differential electrical signals.
2. Discussion of Related Art
There is often a need to measure various parameters of electrical signals. For example, in the manufacture of semiconductor devices, it is desirable to measure parameters of the signals produced by those devices to verify that the devices are operating properly. Information obtained through testing can be used to identify and discard devices that fail to exhibit the expected performance. Test results can sometimes be used to alter the steps in the process used to make the devices. The devices might, for example, be calibrated in subsequent process steps so that they do exhibit expected performance or the devices might be packaged for sale as parts that meet relaxed performance specifications. Alternatively, the results of tests might also be used in a yield enhancement system to change parameters of processing equipment.
As the performance of semiconductor devices has increased, the difficulty of testing those devices has increased. Electronic systems have come to operate at faster and faster speeds. Also, it has become more prevalent to use low voltage differential signals for fast signals. For example, USB 2.0 and Firewire are serial protocols that employ very fast differential signals. Accurately measuring parameters of fast signals, particularly low voltage differential signals, is a challenge.
Reducing the cost of circuitry that can accurately measure a range of differential values of fast signals over a wide bandwidth range is particularly challenging. One example where such test equipment would be desirable is in measuring the “eye pattern” of differential signals. FIG. 1 shows a circuit used for measuring the eye pattern of a differential signal.
Differential signal Sin is applied as an input to test equipment 100. Test equipment 100 could be automatic test equipment, such as is sold by Teradyne, Inc. of Boston, Mass. Digital signal Sin is applied to the input of differential amplifier 116.
Differential signal Sin has two legs, Sin+ and Sin−. The signal is represented as the difference in voltage on these legs. The output of the differential amplifier 116 is a single ended analog signal representing the difference in voltage on legs Sin+ and Sin−.
The output of differential amplifier 116 is applied to HI-LO comparator 110. HI-LO comparator 110 contains two digital outputs, OUTHI and OUTLO. OUTHI is asserted when the input to comparator 110 is above a threshold established by input Vcomp+. OUTLO is asserted when the input to comparator 110 is below a threshold established by input Vcomp−. The comparison is made when the strobe input to the comparator 110 is asserted.
The values of Vcomp+ and Vcomp− are set by control logic 120. The time at which the strobe input is asserted is controlled by timing generator 118, which is also controlled by control logic 120. In automatic test equipment, control logic 120 might include a combination of special purpose hardware and a general purpose digital computer. Operation of control logic 120 can be controlled by software programming.
OUTHI and OUTLO are provided to data analysis circuitry 122. Data analysis circuitry 122 represents a combination of special purpose hardware, such as memories that capture data generated at a high rate, and general purpose computer processors that can be programmed to perform a desired function. Data analysis may be performed on the same computer that is part of control logic 120. In the example of FIG. 1, data analysis circuitry 122 is programmed to produce a plot depicting an eye pattern of a differential signal.
FIG. 2A shows, in idealized form, a differential signal. The legs S+ and S− are shown separately, with the magnitude of the signal represented by the difference between S+ and S−. FIG. 2A shows a periodic digital signal oscillating between a state representing a logic HI and a state representing a logic LO. Region E1 represents a “rising edge” transition and region E2 represents a “falling edge” transition in an interval denoted P1. Regions E′1 and E′2 represent the rising edge transition and the falling edge transition of a subsequent interval P2.
As shown in the idealized waveform of FIG. 2A, the voltages on the legs S+ and S− have well defined values outside the transition regions, resulting in a relatively large voltage, Vpp, representing the signal. There is a relatively long interval PM, during which the signal has sufficient magnitude that a reliable measurement can be made to determine if the signal is HI or LO.
FIG. 2B illustrates a portion of an actual differential signal. The portion pictured in FIG. 2B corresponds to the portion in interval P1 in FIG. 2A. FIG. 2B represents the impact of noise on the signal. Also, FIG. 2B indicates the impact of rise and fall time limitations of the electronic circuitry that generates and transmits the signal. The transitions are not as crisp as shown in FIG. 2A. The overall result is that both the peak voltage, Vpp, and that the interval PM′, over which the state of the signal can be reliably measured are decreased.
FIG. 2B is sometimes called an “eye pattern.” Understanding the shape of the eye pattern is important for designing circuits that operate on the differential signal. For example, the magnitude of Vpp indicates the amount of noise that can be added to the signal before improper operation is likely to result. The interval PM′ indicates the timing precision required to accurately read the signal. Thus, in testing or characterizing semiconductor devices, it is often important to know the eye pattern of the signals produced by the semiconductor device.
FIG. 2C represents plots of measurements that can be made with test system 100 (FIG. 1) from which characteristics of the eye pattern can be observed. The plot is made according to a technique sometimes referred to as an “edge sweep.” The signal to be plotted is applied to the input of test system 100. The strobe signal is timed so that the measurement occurs at a controlled time relative to the start of the interval. The strobe signal causes comparator 110 to compare the input signal to one of its reference values, such as Vcomp+.
The output of the comparator is a data point indicating whether the input signal, at the time determined by the strobe signal, exceeds Vcomp+. If, at the strobe time, the input signal has a value below the value set for Vcomp+, the output of the comparator will be a logic LO. If the input signal is above the value set for Vcomp+, the output of the comparator will be a logic HI at every cycle. Thus, the output of the comparator gives a very course indication of the value of the signal at one instant in time.
If the measurement is repeated with different values of Vcomp+, it is possible to find some value of Vcomp+ at which the output of comparator 110 is HI and another, slightly larger value of Vcomp+ at which the output of comparator 110 is LO. At the time determined by the strobe signal, it could be determined that the value of the signal Sin is between these two values of Vcomp+.
The same process can be repeated with the timing of the strobe signal changed in each repetition to collect data points at different times relative to the start of the interval. By taking data points for a sufficient number of strobe times, the value of the signal can be plotted over the entire interval.
To capture the effects of noise, a set of data points for the same strobe time and value of Vcomp+ must be collected. The test equipment must be configured so that each measurement is made at the same time relative to the start of an interval so that the data points in the set can be averaged. For a signal as shown in FIG. 2A with a repetitive pattern of intervals, the strobe signal can be set to take a measurement during each repetition of the pattern. For example, for the signal in FIG. 2A, a measurement could be made during interval P1, P2, etc. Each measurement is made with the same timing relative to the start of its interval.
Because of the effects of noise, the signal Sin might at any time take on any value in a range of values. As a result, not all of the data points taken with the same value of Vcomp+ and the same strobe time will have the same value. However, the measurements provide information about the signal because, as the value of Vcomp+ changes, the percentage of data points with a HI or LO value will change. For example, a set of data points taken with Vcomp+ well below this range of values will have nearly 100% of the data points with a HI value. A set of data points taken with value Vcomp+ well above this range of values will have nearly 100% of the data points with a LO value. A set of data points taken with Vcomp+ within this range of values will have a mix of HI and LO values, with the relative percentages an indication of the proximity of the value of Vcomp+ to either the top or bottom of the range.
Therefore, a top and bottom and top of the range of signal values can be identified by finding the data sets in which slightly less than 100% of the data points are HI or 100% of the data points are LO. The values of Vcomp+ used to gather the data points within these data sets define the range boundaries. The value of the signal, including the effects of noise, can be represented at any given time by the bottom and the top of the range.
FIG. 2C depicts a series of four plots that are made by this method. Plots L1 and L2 are made with data taken during rising edges of the signal, such as E1 and E′1. Plots L3 and L4 are with data taken during a falling edges, such as E2 and E′2. Plots L2 and L3 define the lower end of the range. Plots L1 and L4 defined the higher end of the range.
Plots L2 and L3 and plots L1 and L4 are not shown to be connected. FIG. 2C shows only the data collected during times corresponding to rising or falling edges of the signal. Similar data for the entire interval could be collected and presented, in which case the plots L2 and L3 and plots L1 and L4 would appear connected.
The plots L1, L2, L3 and L4 include both positive and negative values because a “rising edge” could represent a LO to HI transition of the signal, such as is depicted at E1 or a HI to LO transition, such as depicted at E′1.
While making a plot such as shown in FIG. 2C is desirable, constructing a differential amplifier such as 116 for use in a test system can be difficult and expensive. To make an accurate measurement on the signal, the amplifier must output an accurate representation of the input signal. The amplifier must have a precisely controlled gain over a broad range of operating frequencies.